ACR Y HAD LTSS FROM ACR X DURING LNDG ON OCCUPIED RWY. SYS ERROR.

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DURING THE APCH; ACR X TOLD SEVERAL TIMES OF ACFT LNDG ON 22L THAT WOULD HOLD SHORT OF OUR RWY. DURING ROLLOUT; AFTER A NORMAL LNDG ON RWY 27 IN BOSTON; OUR ACFT STARTED SLIDING AND THE ANTI-SKID CYCLING WHEN WE REACHED THE NORMAL TURN OFF AREA. THE RWY WAS WET AND A MODERATE XWIND; PLUS THE RUBBER LAID DOWN BY PRIOR LNDGS AND ACFT MAKING THE TURN OFF; CONTRIBUTED TO THE SITUATION. THE RESULT WAS THAT OUR ACFT PENETRATED THE 1770 FT STOPWAY BY ABOUT 1 ACFT LENGTH OR 100 FT. THE TWR WAS TELLING US TO TURN OFF AT X OR S; BUT WE WERE UNABLE TO DUE TO OUR SLIDING. AFTER WE STOPPED; THE TWR SAID TO TURN L ON S OR S1; BUT THESE TAXIWAYS WERE SLIGHTLY BEHIND US. ALSO; THE L TURN DIRECTION CONFUSED ME BECAUSE VIRTUALLY THE ENTIRE ARPT WAS TO OUR R; AND I THOUGHT THE CTLR HAD SAID L BY MISTAKE. THEREFORE; I REMAINED STOPPED IN THE STOPWAY UNTIL THE RADIO TFC DIED DOWN AND WE COULD GET SPECIFIC AND CLR INSTRUCTIONS. WHILE STOPPED; I OBSERVED AN ACFT TKOF FROM 22R AND PASS OVER AND IN FRONT OF US AT ABOUT A 30 DEG UP ANGLE AND 300 FT AWAY. THEN THE TWR TOLD US TO PROCEED AHEAD AND TURN R ON B. WHEN I TURNED OFF; I SAW AN ACR Y ON LNDG ROLL; ABOUT HALF WAY DOWN RWY 27 WITH SPRAY FLYING FROM HIS REVERSING. HE MUST HAVE LANDED WHILE WE WERE IN THE STOPWAY. SEVERAL HUMAN PERFORMANCE FACTORS ARE HIGHLIGHTED BY THIS INCIDENT: I SHOULD HAVE TAKEN CTL OF THE ACFT SOONER; BUT I WAS RELUCTANT TO DO SO; SINCE I CONSIDERED MY FO TO BE EXTREMELY COMPETENT. TWR CTLRS SHOULD UNDERSTAND THAT PLTS OPERATE INTO NUMEROUS ARPTS; AND CONSEQUENTLY CANNOT BE EXPECTED TO KNOW EVERY TAXIWAY AT EVERY ARPT BY NAME; LIKE THE CTLR WHO WORK THE SAME ARPT EVERYDAY. ADDITIONALLY; DURING LNDG ROLLOUT AND TRYING TO CLR A RWY EXPEDITIOUSLY IS NOT THE TIME TO GO HEADS DOWN IN THE COCKPIT TO LOOK AT AN ARPT DIAGRAM. A BETTER CALL WOULD BE 'TURN R AT THE SECOND TAXIWAY IF ABLE.' PLTS SHOULD BE ENCOURAGED NOT TO ANSWER THESE CALLS UNTIL TAXI SPD (10-15 KTS) HAS BEEN ACHIEVED. UNTIL THAT TIME; THE PLT SHOULD HAVE FULL USE OF THE RWY TO INCLUDE ANY STOPWAY. NO ACFT SHOULD BE ALLOWED TO LAND BEHIND AN ACFT THAT IS IN A STOPWAY IN CASE THE LNDG ACFT ALSO NEEDS THE STOPWAY. NO ACFT SHOULD BE GIVEN TKOF CLRNC WHEN HIS RWY INCLUDING STOPWAY; INTERSECTS A LNDG ACFT'S RWY INCLUDING STOPWAY; UNTIL THE LNDG ACFT IS AT TAXI SPD. IN THIS CASE; HAD THE ACR Y ON RWY 27 GONE AROUND; HE MAY HAVE CONFLICTED WITH THE ACFT TAKING OFF ON RWY 22R. IN THEIR PRAGMATIC APCH TO LAUNCH AND RECOVER ACFT AS JUDICIOUSLY AS POSSIBLE; NO CTLING AGENCY SHOULD ALLOW A SITUATION TO DEVELOP; WHEREBY AN UNANTICIPATED OCCURRENCE CAUSES ANY OTHER ACFT TO NOT HAVE A SAFE ESCAPE RTE. SUPPLEMENTAL INFO FROM ACN 190017. FLT WAS UNEVENTFUL THROUGH APCH AND TOUCHDOWN. UNTIL APPROX 50 KIAS; TRACTION AND DECELERATION WERE NORMAL DESPITE MODERATELY HVY WT DUE TO PAX LOAD AND FUEL TANKERING. WITH APPROX 1000-1200 FT REMAINING BEFORE THE 1770 FT STOPWAY; THE ANTI-SKID SYS BEGAN CYCLING. WITH THE ANTI-SKID CYCLING; THE RATE OF DECELERATION WAS STILL SATISFACTORY TO TURN ON TAXIWAY S AS DIRECTED BY THE TWR. WITH LESS THAN 1000 FT REMAINING; THE RATE OF DECELERATION DECREASED SIGNIFICANTLY. WHEN WHEEL BRAKE TRACTION BECAME VERY LOW; THE ACFT BEGAN WEATHERVANING TO THE L. WIND WAS RPTED AS 230 DEG AT 11 KTS. WITH THE NOSE OF THE ACFT BEGINNING TO DRIFT L; I HAD 2 CHOICES: I COULD EITHER CONTINUE WITH THE SAME BRAKE; RUDDER; AND NWS INPUTS AND HOPE THE TRACTION RETURNED BEFORE THE NOSE DRIFTED TOO FAR L; OR I COULD RELEASE THE L BRAKE (FURTHER SLOWING THE ACFT'S OVERALL DECELERATION) AND STRAIGHTEN OUT THE NOSE. WITH THE STOPWAY AREA PRESENTING A SAFE; WT BEARING 1770 FT SURFACE DIRECTLY IN FRONT OF THE ACFT; I CHOSE THE SECOND OPTION. IMMEDIATELY AFTER RELEASING THE L BRAKE AND STRAIGHTENING THE NOSE; THE CAPT TOOK CTL OF THE ACFT AND CAME TO A STOP APPROX 100 FT INTO THE STOPWAY. CONTRIBUTING FACTORS WERE WET RWY; MODERATE XWIND; GROSS WT; RUBBER DEPOSITS ON THE RWY AT THE END OF RWY 27; AND RWY LENGTH. BUT THE PRIMARY FACTOR WAS A LACK OF COGNIZANCE ON MY PART THAT USE OF THE STOPWAY WOULD BE TREATED AS A 'RWY OVERRUN' VERSUS BEING TREATED AS USE OF AN AVAILABLE AREA SIMILAR TO A DISPLACED THRESHOLD WHICH IS DESIGNED TO BE USED IF NEEDED. THE SITUATION WOULD HAVE BEEN PREVENTED IF SLICK RWY ADVISORIES HAD BEEN GIVEN OR IF RWY MARKINGS HAD NOT BEEN SIMILAR TO DISPLACED THRESHOLD MARKINGS (YELLOW CHEVRONS). ALSO WITH THE TWR ADVISING US MULTIPLE TIMES THAT OTHER ACFT WOULD HOLD SHORT OF OUR RWY; IT FURTHER INDICATED THAT ALL AVAILABLE ROLLOUT AREA WAS FREE FOR OUR USE.